1. Technical Field
Embodiments of the subject matter disclosed herein generally relate to devices and methods for generating seismic waves underground and, more particularly, to mechanisms and techniques for generating seismic waves with volumetric and non-volumetric seismic sources.
2. Discussion of the Background
Land seismic sources may be used to generate seismic waves in underground formations for investigating geological structures. A seismic source may be located on the ground or it may be buried in the ground. The seismic source, when activated, imparts energy into the ground. Part of that energy travels downward and interacts with the various underground layers. At each interface between these layers, part of the energy is reflected and part of the energy is transmitted to deeper layers. The reflected energy travels toward the surface of the earth, where it is recorded by seismic sensors. Based on the recorded seismic data (traces), images of the underground layers may be generated. Those skilled in the art of seismic image interpretation are then able to estimate whether oil and/or gas reservoirs are present underground. A seismic survey investigating underground structures may be performed on land or water.
Current land seismic sources generate a mixture of P-waves and S-waves. A P-wave (or primary wave or longitudinal wave) is a wave that propagates through the medium using a compression mechanism, i.e., a particle of the medium moves parallel to a propagation direction of the wave and transmits its movement to a next particle of the medium. This mechanism is capable of transmitting energy both in a solid medium (e.g., earth) and in a fluid medium (e.g., water). An S-wave, different from a P-wave, propagates through the medium using a shearing mechanism, i.e., a particle of the medium moves perpendicular to the propagation direction of the wave and shears the medium. This particle makes the neighboring particle also move perpendicular to the wave's propagation direction. This mechanism is incapable of transmitting energy in a fluid medium, such as water, because there is not a strong bond between neighboring water particles. Thus, S-waves propagate only in a solid medium, i.e., earth.
The two kinds of waves propagate with different speeds, with P-waves being faster than S-waves. They may carry different information regarding the subsurface and, thus, both are useful for generating a subsurface image. However, when both of them are recorded with the same receiver, the strong S-wave content may obscure the P-wave content in certain portions, rendering the final image inaccurate.
Thus, there is a need to record both types of waves, with the ability to separate, at the emission stage, the two kinds of waves as needed. However, current use of land seismic sources does not offer this possibility. Currently, P- and S-waves generated by a land seismic source are simultaneously recorded by plural receivers, and during the processing stage, various strategies are employed for separating the two. However, this process may be time-intensive and inaccurate.